Wavelength locked fiber-coupled diode-laser bar

ABSTRACT

In apparatus for wavelength stabilizing and spectrally narrowing an output beam of a diode-laser, a cylindrical lens is arranged to collimate the beam in the fast axis of the diode laser without reducing divergence in the slow axis of the diode-laser. A length of optical fiber is arranged to receive the fast-axis collimated beam from the lens. The optical fiber has a core surrounded by a first cladding, the first cladding being surrounded by a second cladding. The core of the optical fiber has an elongated cross section and includes a wavelength selective Bragg grating. The core functions as a low-mode core in the width direction of the cross section. The length direction of the core is aligned with the fast axis of the diode-laser. The fast-axis collimated beam and the low mode width of the core provides that the Bragg grating only reflects light that propagates parallel the longitudinal axis of the fiber. Light reflected from the grating is fed back to the diode-laser for stabilizing the wavelength and spectrally narrowing the diode-laser output beam.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to stabilizing the output of alinear diode-laser array or diode-laser bar. The invention relates inparticular to stabilizing the output of a diode-laser bar when thatoutput is coupled into an optical fiber array.

DISCUSSION OF BACKGROUND ART

Diode-laser light is commonly used for optically pumping solid-statelasers and fiber lasers. As light from a single diode-laser is ofteninsufficiently powerful for such pumping, it is usual to use light froma plurality of diode-lasers arranged in a linear array. Such an array iscommonly referred to by practitioners of the art as a diode-laser bar.The light from the diode-lasers in the bar must be collected by anoptical arrangement that makes the sum of the outputs of thediode-lasers available for pumping.

Light is emitted from a diode-laser as a diverging beam. The beamdiverges strongly, for example at about 35 degrees, in one axis, termedthe fast axis, and diverges weakly, for example at about 10 degrees, inan axis (the slow axis) perpendicular to the fast axis. In a diode-laserbar individual diode-lasers (emitters) are arranged, spaced apart,linearly, in the slow axis direction. In a high power diode-laser of thetype used for optical pump light is emitted in a plurality of modes(multimode output). A preferred method of collecting the outputs of theplurality of emitters of a diode-laser bar is to couple the individualemitter outputs into a corresponding plurality of multimode opticalfibers having entrance ends thereof arranged in a linear array alignedwith the slow axis of the diode laser bar. A cylindrical microlens isused to collimate the emitter output in the fast axis the output iscoupled into the fibers. Output ends of the optical fibers are formedinto a bundle. Light output from the bundle can be collected by a lensand focused directly into a solid-state gain medium or into a singleoptical fiber, which can be a transport fiber or a fiber laser to bepumped.

In the gain medium of a solid-state laser or a fiber laser absorption ofoptical pump light can often only occur in a narrow band of wavelengths,for example, about 1 nanometer (nm) wide. This narrow band ofwavelengths is centered on a fixed, peak absorption wavelength that ischaracteristic of the gain medium. Absent any constraint, diode-laserlight is emitted in a relatively broad spectrum of wavelengths, forexample between 2 nm and 5 nm. Accordingly, for optimizing opticalpumping efficiency, it is preferable to provide a constraint thatnarrows the emission bandwidth and stabilizes the center wavelength ofthis narrowed emission bandwidth at the characteristic peak absorptionwavelength of the gain medium.

One arrangement that has been used to narrow the bandwidth and stabilizethe wavelength of the output of a single mode diode-laser coupled intothe core of a single mode fiber, is to write a fiber Bragg grating intothe core of the fiber. The refractive index modulation and the period ofmodulation of the Bragg grating are selected such that the gratingreflects back into the diode-laser a few percent (usually less than 10%)of radiation propagating in the core in a bandwidth less than about 1 nmabout a peak-reflection wavelength determined by modulation period. Thisforces the diode laser to emit at the peak-reflection wavelength of thegrating and with a bandwidth about equal to the reflection bandwidth ofthe grating.

This method however is not suitable for use with multimode fibers sincea multimode fiber supports all the directions (angles) of propagationwithin its numerical aperture. The peak reflection wavelength of a fiberBragg grating depends not only on the modulation period but on the angleof incidence of light on the grating. Accordingly, in a multimode fibercore, a Bragg grating would have a reflection bandwidth broadened by theplurality of angles at which the multimode light was incident thereon.The grating would provide neither adequate bandwidth narrowing noradequate wavelength stabilization. There is a need for an arrangementfor stabilizing the wavelength and narrowing the bandwidth of the outputof a multimode fiber coupled, multimode diode-laser bar.

SUMMARY OF THE INVENTION

In one aspect, the present invention comprises a diode-laser emitteremitting an output beam along a propagation axis. The diode-laser has afast axis and a slow axis perpendicular to each other and perpendicularto the propagation axis. The emitted beam diverges in the fast axis andin the slow axis, with the slow axis divergence being greater than thefast axis divergence. A lens is disposed on the propagation axis of thediode-laser. The lens is configured and aligned with the diode-lasersuch that the output beam is transmitted by said lens collimated in thefast axis and the slow axis divergence unchanged. A length of opticalfiber is arranged to receive the transmitted output beam. The opticalfiber has a core surrounded by a first cladding, the first claddingbeing surrounded by a second cladding. The first cladding has a lowerrefractive index than that of the core, and the second cladding has alower refractive index than that of the first cladding. The core of theoptical fiber includes a wavelength selective Bragg grating and has anelongated cross-section. The width of the cross-section is selected suchthat the core functions as a low-mode core in the width direction of thecross section. The length direction of the core is aligned with the fastaxis of the diode-laser.

The fast axis collimated beam, together with cross-section shape of thecore and the arrangement thereof with respect to the fast axis of thediode-laser, provides that the Bragg grating will only reflect the lightthat propagates along or parallel to the axis of the fiber. Tilted raysin the slow axis will not have sufficient path length in the core tointeract effectively with the grating. This provides that the gratingreflects in a bandwidth only about 1 nm or less back into thediode-laser thereby providing the desired spectral narrowing andwavelength stabilization of the diode-laser. Multimode light willpropagate in the first cladding but having the stabilized wavelength andnarrow bandwidth characteristics forced by the back reflection from thegrating.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain the principles of the presentinvention.

FIGS. 1A and 1B are plan and elevation views schematically illustratingone embodiment of diode-laser apparatus in accordance with the presentinvention including a diode-laser bar having a plurality of emitters anda cylindrical lens arranged to couple output of the emitters into acorresponding plurality of double-clad optic fibers each thereofincluding a core having an elongated cross-section shape and having aBragg grating written therein, the cross section shape being configuredand arranged such that the core functions as a multimode core in thefast axis of the emitters and a low-mode core in the slow axis of theemitters.

FIG. 2 is end elevation view schematically illustrating detail of onepreferred example of a double-clad optical fiber in the apparatus ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like features are designated bylike reference numerals, FIG. 1A and FIG. 1B schematically illustrate apreferred embodiment 10 of a fiber-coupled diode-laser bar apparatus inaccordance with the present invention. Apparatus 10 includes adiode-laser bar 12 including a plurality of spaced-apart individualemitters (diode-lasers) 14 arranged in a linear array. The diode-laserbar is mounted on a heat-sink 16.

Each emitter 14 in diode-laser bar 12 has a fast axis and a slow axis(Y-axis and X-axis respectively as depicted in FIGS. 1A-B). Theindividual emitters are aligned in the slow axis. Each emitter emits abeam of light 18 generally along a propagation axis 20 (the Z-axis asdepicted in FIGS. 1A-B). Beam 18 diverges in the slow axis and the fastaxis with the fast axis divergence being greater as discussed above. InFIG. 1A beam 18 is depicted by extreme slow-axis diverging rays 18S. InFIG. 1B beam 18 is depicted by extreme fast-axis diverging rays 18F.

A cylindrical microlens (lens) 22 is disposed on propagation axis 20 ofthe diode-lasers to receive and transmit the emitted beams. The lens isarranged to at least reduce the divergence of the fast axis rays ontransmission and preferably to collimate the fast axis rays ontransmission as depicted in FIG. 1B. The divergence of slow axis rays18S is unchanged on transmission through lens 22.

Continuing with reference to FIGS. 1A and 1B and with reference inaddition to FIG. 2, a plurality of double-clad optical fibers 24, equalin number to the number of emitter 14 in diode-laser bar 12, is arrangedsuch that each one of the plurality of fibers a beam 18 from acorresponding one of the diode-lasers after the beam has beentransmitted, and fast-axis collimated, by lens 18. Each fiber 24 has acore 26 having an elongated, here elliptical, cross-section (see FIG.2). The core is “low-mode” in the width direction of core cross-section,that is to say the width (here the minor axis) W of the core issufficiently narrow so only about 4 or less modes would be supportedwere the core circular (with a diameter W). A single-mode core, ofcourse, would be a limiting case of a low-mode core. Core 26 is“multimode” in the length direction of the core cross-section, that isto say, the length (here the major axis) L of the core is sufficientlylong that tens or even a hundred or more modes could be supported werethe core circular (with a diameter L). By way of example, for lighthaving a wavelength of about 808 nm, W is preferably about 30.0micrometers (μm) or less and L is preferably about 50.0 μm or greater orgreater. Core 26 has a Bragg grating 32 written into the core. Therefractive index modulation depth of the grating is preferably selectedsuch that the grating has a reflection bandwidth less than the gainbandwidth of the diode-laser and preferably less than about 1 nm. Thegrating period is selected such that the grating has a peak reflectivityat a wavelength within the gain-bandwidth of the diode-laser at whichthe output of the diode-laser is to be stabilized. The length of thegrating is selected, consistent with the refractive index modulation,according to the reflectivity desired for the grating.

Core 26 is surrounded by a cladding 28 having a refractive index lowerthan the refractive index of the core. Cladding 28 is surrounded by afurther cladding 30 having a refractive index lower than the refractiveindex of cladding 28. In one example of a fiber 24 for receiving 808 nmlight core 26 has a length L of about 100.0 μm, and a width W of about30.0 μm. Cladding 28 has a diameter of about 150.0 μm and cladding 30has a diameter of about 165.0 μm. Note that the cladding 28 acts as acore for light whose divergence is too large to be accepted into thesmaller core 26.

Each fiber 24 is arranged such that the length direction of thecross-section of core 26 thereof is aligned with the fast axis of thecorresponding emitter 14. Any fast axis rays entering the core, havingbeen collimated by lens 22, travel along or parallel to the longitudinalaxis of the fiber. As core 26 has only a relatively narrow widthcompared to the beam width in the slow axis of an emitter, only thoseslow-axis rays propagating along or close to the fiber axis will entercore 26. Those slow axis rays that are not propagating exactly along thefiber axis will have too short an interaction with the grating to haveany significant bandwidth broadening effect on back reflection from thegrating. The configuration and arrangement of fiber 24 provides thatessentially all rays interacting with grating 32 of core 26 interactwith the grating at the same angle, i.e., paraxial to the longitudinalaxis of the fiber, at about normal incidence to the grating. Note thatthis is true in the fast axis direction, even thought the core is longersince the rays in fast axis have been collimated by the lends. Thisapproach the bandwidth broadening of a grating reflection band thatwould occur if the grating were written into a conventional multimodefiber core.

It is emphasized, here, that all rays in beam 18 that are within thenumerical aperture (NA) of fiber 24 defined by the refractive indexdifference of inner cladding 28 and inner cladding 30 propagate in thecladding 28. Preferably the NA of fiber as so defined is about 0.15 orgreater, for accepting the slow axis ray divergence. Fast axis rays, ofcourse, are collimated. Most of the rays entering the fiber willpropagate in inner cladding 28 thereof, while a small percentage, forexample between about 10% and 20%, having characteristics discussedabove will propagate in the core 26 and interact with grating 32. The NAof a fiber 24 as defined by the refractive index difference between core26 and cladding 28 can be as low as about 0.07 or less, as the core isintended to carry paraxial rays in the fiber as discussed above.

It will be evident from the description provided above that innercladding 28 can be considered the “multimode core” of fiber 24 as far asmultimode beam transport aspects of the fiber are concerned. Core 26serves only to carry the grating 32 and carry a small percentage of raysentering the fiber for providing back reflection into an emitter.Accordingly, a fiber 24 need be no longer than is necessary to provide asufficient length of grating 32 to provide a desired reflectivity of thegrating. By way of example a length of about 5.0 cm may be sufficient.This short length of fiber may then be spliced to a conventionalmultimode fiber for further transport of the emitter output. An exampleof this is depicted in FIGS. 1A and 1B wherein one fiber 24 is depictedas having a multimode fiber 34 spliced thereto. Fiber 34 has a multimodecore 36 having about the same diameter as inner cladding 28 of fiber 24.Fiber 34 has a cladding 38 having about the same thickness as outercladding 30 of fiber 24.

It may be possible to configure the core 26 as a conventional circularsingle mode core having a grating written therein. However, it isbelieved that the single mode core would intercept such a small fractionof the total beam that insufficient feedback would be provided even ifthe grating therein had a reflectivity of 100%. By elongating the corecross-section in the fast-axis direction, a sufficient proportion of thebeam can be intercepted to provide adequate feedback with a gratinghaving less than 100% reflectivity.

Regarding the reflectivity of grating 32, this should be significantlyhigher than would be the case in prior-art grating feedback practice aseven with the elongated core 26 only a small percentage of the beaminteracts with the grating as discussed above. Preferably, the gratinghas a reflectivity of about 50% percent or greater. This preferenceassumes that facet reflectivity of the emitters is between about 1% and10% percent. Optimization of the grating reflectivity and reflection ofoutput facets of the emitters for any particular case can be simply doneby experiment. Many diode-laser bar chips can be fabricated in a singlegrowth cycle. These can be provided with a range of output facetcoatings for providing a range of facet reflectivities. Fibers 24 havinggratings 32 of different reflectivity can be prepared and variouscombinations of diode-laser and fiber can be evaluated.

The present invention is described above as a preferred embodiment. Theinvention, however, is not limited to the embodiment described anddepicted. Rather, the invention is limited only to the claims appendedhereto.

1. Optical apparatus comprising: a diode-laser emitter arranged to emitan output beam along a propagation axis, said diode-laser having fastaxis, and a slow axis perpendicular to each other and perpendicular tosaid propagation axis, said beam having a first divergence in said fastaxis and a second divergence in said slow axis, said second divergencebeing less than said first divergence, and said diode-laser having again-bandwidth; a lens disposed on said propagation axis of saiddiode-laser, said lens configured and aligned with said diode-laser suchthat said output beam is transmitted by said lens collimated in saidfast axis and with said slow axis divergence unchanged; a length ofoptical fiber arranged to receive said transmitted output beam, saidoptical fiber having a core surrounded by a first cladding, said firstcladding being surrounded by a second cladding, said first claddinghaving a lower refractive index than that of said core, and said secondcladding having a lower refractive index than that of said firstcladding; said core including a wavelength-selective Bragg grating andhaving an elongated cross-section, said cross-section having a widthselected such that said core functions as a low-mode core in said widthdirection of said cross section; wherein said length direction of saidcore is aligned with said fast axis of said diode-laser and wherein saiddiode-laser has a gain-bandwidth and said Bragg Grating has a reflectionbandwidth less than said gain bandwidth and has a peak reflectivitywithin said gain-bandwidth.
 2. The apparatus of claim 1, wherein saidBragg grating has a reflectivity greater than about 50%.
 3. Theapparatus of claim 1, wherein said core has an elliptical cross-section,said length and width thereof corresponding respectively to major andminor axes of said elliptical cross-section.
 4. The apparatus of claim3, wherein said width of said core is about 30 microns or less, and saidlength of said core is about 50 microns or greater.
 5. The apparatus ofclaim 1, wherein said optical fiber has a first numerical aperturedefined by the refractive indices of said first and second cladding, andsaid numerical aperture is about 0.15 or greater.
 6. The apparatus ofclaim 1, wherein said optical fiber has a second numerical aperturedefined by the refractive indices of said core and said first claddingand said second numerical aperture is about 0.07 or less.
 7. Opticalapparatus comprising: a diode-laser emitter having a gain bandwidth andarranged to emit an output beam along a propagation axis, saiddiode-laser having fast axis, and a slow axis perpendicular to eachother and perpendicular to said propagation axis, and said beam having afirst divergence in said fast axis and a second divergence in said slowaxis, said second divergence being less than said first divergence; alens disposed on said propagation axis of said diode-laser, said lensconfigured and aligned with said diode-laser such that said output beamis transmitted by said lens collimated in said fast axis and with saidslow axis divergence unchanged; a first optical fiber arranged toreceive said transmitted output beam, said optical fiber having a coresurrounded by a first cladding, said first cladding being surrounded bya second cladding, said first cladding having a lower refractive indexthan that of said core, and said second cladding having a lowerrefractive index than that of said first cladding; said core including awavelength-selective Bragg grating having a reflection bandwidth lessthan the gain bandwidth of said diode-laser and a peak reflection at awavelength within said gain-bandwidth; said core having an elongatedcross-section, said cross-section having a width selected such that saidcore functions as a low-mode core in said width direction of said crosssection, and said length direction of said core being aligned with saidfast axis of said diode-laser; and wherein said diode-laser, said lens,and said optical fiber are so arranged that a first portion of saiddiode-laser output beam transmitted by said lens propagates in saidfirst cladding of said optical fiber, a second portion of saiddiode-laser output beam transmitted by said lens is intercepted by saidcore, and a portion of said second portion of said beam is reflected bysaid Bragg grating, via said lens, back to said diode-laser.
 8. Theapparatus of claim 7, wherein said second portion of said diode-laseroutput beam is between about 10% and 20%.
 9. The apparatus of claim 7,wherein said Bragg grating has a peak reflectivity of about 50% orgreater.
 10. The apparatus of claim 7, further including a secondoptical fiber spliced to an exit face of said first optical fiber, saidsecond optical fiber having a multimode core surrounded by a secondcladding.
 11. The apparatus of claim 10, wherein said multimode core ofsaid second optical fiber has a diameter about equal to the diameter ofthe first cladding of said first optical fiber.
 12. The apparatus ofclaim 7, wherein said optical fiber has a first numerical aperturedefined by the refractive indices of said first and second cladding, andsaid numerical aperture is about 0.15 or greater.
 13. The apparatus ofclaim 7, wherein said optical fiber has a second numerical aperturedefined by the refractive indices of said core and said first claddingand said second numerical aperture is about 0.7 or less.
 14. Opticalapparatus comprising: a diode-laser emitter having a fast axis and aslow axis perpendicular thereto, with light in the fast axis divergingfaster than in the slow axis; a cylindrical lens positioned tosubstantially collimate light emitted along the fast axis; an opticalfiber positioned to receive light transmitted by said lens, said opticalfiber having a core surrounded by a first cladding, said first claddingbeing surrounded by a second cladding, said first cladding having alower refractive index than that of said core, and said second claddinghaving a lower refractive index than that of said first cladding, saidcore including a wavelength-selective Bragg grating; and with thediameter of the core that is aligned with the slow axis being selectedto substantially limit the cone of light accepted to propagatetherethrough of those rays traveling in the slow axis so that thebandwidth of the light reflected from the Bragg grating back to thediode-laser emitter is substantially reduced and wherein the diameter ofthe core aligned with the fast axis is greater than the diameter of thecore aligned with the slow axis direction.
 15. An optical apparatus asrecited in claim 14, wherein the diameter of the core is configured sothat only about 20% or less of the light in the slow axis propagates inthe core.
 16. Optical apparatus comprising: a diode-laser emitterarranged to emit an output beam along a propagation axis, saiddiode-laser having fast axis, and a slow axis perpendicular to eachother and perpendicular to said propagation axis, said beam having afirst divergence in said fast axis and a second divergence in said slowaxis, said second divergence being less than said first divergence, andsaid diode-laser having a gain-bandwidth; a lens disposed on saidpropagation axis of said diode-laser, said lens configured and alignedwith said diode-laser such that said output beam is transmitted by saidlens collimated in said fast axis and with said slow axis divergenceunchanged; a length of optical fiber arranged to receive saidtransmitted output beam, said optical fiber having a core surrounded bya first cladding, said first cladding being surrounded by a secondcladding, said first cladding having a lower refractive index than thatof said core, and said second cladding having a lower refractive indexthan that of said first cladding; said core including awavelength-selective Bragg grating and having an elongatedcross-section, said cross-section having a width selected such that saidcore functions as a low-mode core in said width direction of said crosssection; and wherein said length direction of said core is aligned withsaid fast axis of said diode-laser and wherein said optical fiber has afirst numerical aperture defined by the refractive indices of said firstand second cladding, and said numerical aperture is about 0.15 orgreater.
 17. Optical apparatus comprising: a diode-laser emitterarranged to emit an output beam along a propagation axis, saiddiode-laser having fast axis, and a slow axis perpendicular to eachother and perpendicular to said propagation axis, said beam having afirst divergence in said fast axis and a second divergence in said slowaxis, said second divergence being less than said first divergence, andsaid diode-laser having a gain-bandwidth; a lens disposed on saidpropagation axis of said diode-laser, said lens configured and alignedwith said diode-laser such that said output beam is transmitted by saidlens collimated in said fast axis and with said slow axis divergenceunchanged; a length of optical fiber arranged to receive saidtransmitted output beam, said optical fiber having a core surrounded bya first cladding, said first cladding being surrounded by a secondcladding, said first cladding having a lower refractive index than thatof said core, and said second cladding having a lower refractive indexthan that of said first cladding; said core including awavelength-selective Bragg grating and having an elongatedcross-section, said cross-section having a width selected such that saidcore functions as a low-mode core in said width direction of said crosssection; and wherein said length direction of said core is aligned withsaid fast axis of said diode-laser and wherein said optical fiber has asecond numerical aperture defined by the refractive indices of said coreand said first cladding and said second numerical aperture is about 0.07or less.